(a) Field of the Invention
This invention relates to exhaust gas purification catalytic materials and methods for fabricating the same.
(b) Description of Related Art
Catalytic materials are generally formed so that a precious metal catalyst component is loaded on a support material or materials by impregnation or other processes. For example, three-way catalytic converters for purifying automotive exhaust gas emissions are formed by coating a substrate with alumina and ceria serving as support materials to form a washcoat layer, impregnating the washcoat layer with Pt, Pd, Rh or other precious metals and then calcining the washcoat layer. Ceria acts, in a three-way catalyst, as an oxygen storage component to expand the range of engine air-to-fuel (A/F) ratios within which the catalyst can act effectively. However, ceria has a problem of ineffectiveness in improvement of catalyst thermal resistance.
More specifically, in recent years, in order to effectively purify HC and other components in exhaust gas even at engine start or at low outside temperatures, catalytic converters have been placed closer to the engine to early raise their temperature. Therefore, and also because of increased engine power output, catalysts have more frequently been exposed to high temperatures. However, ceria itself has a poor thermal resistance, which allows sintering of a precious metal catalyst component to early deteriorate the catalyst. Though a simple solution to this problem is to increase the amount of precious metal catalyst component, it leads to increased catalyst cost.
As another solution to the above problem, Japanese Unexamined Patent Publication No. 10-182155 discloses a technique in which alumina-ceria-zirconia mixed oxide having oxygen storage capacity is used as a support material for a precious metal catalyst component to improve the catalyst thermal resistance. The mixed oxide is obtained by preparing an acidic solution containing Ce, Zr and Al ions, pouring this acidic solution and aqueous ammonia concurrently onto a rotating disc to evenly mix both the liquids within one second and thereby produce a mixed oxide precursor, and subjecting the precursor to drying and calcination.
Alternatively, Japanese Unexamined Patent Publication No. 2000-300989 discloses a catalytic material (oxide) containing Ce, Zr and a precious metal catalyst component. This catalytic material is obtained by adding aqueous ammonia to a mixture of cerium nitrate solution, zirconium nitrate solution and precious metal solution (e.g., palladium nitrate solution), filtering a precipitate obtained by coprecipitation, subjecting the precipitate to rinsing and drying, and then calcining it.
In the former technique, when the mixing of the acidic solution and aqueous ammonia is completed in a short time, the dispersivity of metal components is increased to improve the oxygen storage capacity of the resultant mixed oxide, but there is an inevitable limit to such improvement in oxygen storage capacity. Therefore, in order to improve the catalyst performance, it is necessary to load a relatively large amount of precious metal catalyst component on the mixed oxide.
In the latter technique of obtaining a catalytic material (oxide) containing Ce, Zr and a precious metal catalyst component by coprecipitation, aqueous ammonia is dropped into the mixed acidic solution. As a result, the precipitate is separated into early and later precipitated substances and thus does not become a homogeneous precipitate. Therefore, according to this technique, it is difficult to improve the performance of the resultant catalytic material.